The research proposed seeks to elucidate the structure and function of iron-containing proteins. It is also proposed to explore the electronic structure of synthetic complexes which mimick structural features of the metal centers observed in the proteins. The experimental techniques employed are 57Fe Mossbauer spectroscopy and Electron Paramagnetic Resonance (EPR). (1) Studies of the binuclear clusters of the iron-oxo proteins methane monooxygenase and ribonucleotide reductase will be continued, with focus on the electronic structure of the FeIIFeII cluster forms and intermediates of the oxygen reactions. (2) This research will be complemented by studies of homonuclear and heteronuclear model complexes of iron-oxo proteins. (3) Mossbauer measurements to explore the structure and chemistry of the dioxygen reduction site of cytochrome ba3 from Thermus thermophilus will be continued. Efforts will be made to characterize the electronic structure of the coupled cytochrome a3/CuB site and to search for intermediates of the catalytic reaction. (4) Recently begun studies of novel FeIV complexes will be continued and expanded. (5) It is also proposed to study complexes which contain either an Fe or a Ni site linked by a bridging ligand to an Fe4S4 cluster. Such complexes are interesting models for the active sites of E. coli sulfite reductase and, perhaps, CO dehydrogenase. (6) Studies of CO dehydrogenase and isopenicillin N synthase will be continued. (7) Finally, we will continue to develop integer spin EPR with the goal of further improving the quantitative aspects of this new technique.